Instrumentation slide 7

1. Strain Gauges and its applications
in measurement system

2. 1. Understand the basic operating principle of a strain gauge
2. Able to install a strain gauge with correct procedure
3. Able to apply strain gauge for different measurement
procedures.
4. Able to understand and apply Wheatstone bridge circuit for
strain gauge measurement

3. RECAP ON STRESS AND STRAIN

4. • Find the strain that results from a tensile force of 1000 N applied to a 10-m aluminum
beam having a m2 cross-sectional area. Given that the Young’s Modulus foran
aluminum is N/m2
4
4 10−
×
10
6.89 10
×

5. • The strain gauge is a passive,
resistive transducer which
converts the mechanical
elongation and compression
into a resistance change.
• This change in resistance takes
place due to variation in length
and cross sectional area of the
gauge wire, when an external
force acts on it.
STRAIN GAUGES

6. STRAIN GAUGES
• A Strain Gauge is a device used to measure the strain of an object
• The gauge is attached to the object by a suitable adhesive
• As the object is deformed, the foil is deformed, causing its electrical resistance to change
• The resistance change is commonly measured using a Wheatstone bridge
• The most common type of strain gauge consists of an insulating flexible backing which
supports a metallic foil pattern
2
where Sample Resistance ( )
Sample Resistivity ( .m)
length in (m)
cross-sectional area in (m )
l
R
A
R
l
A
ρ
ρ
=
Ω
Ω
=
=

7. • Determine the overall resistance for a copper wire with diameter 1mm, and length 5cm.
Resistivity of copper is 8
1.7 10 m
−
× Ω
( )
( )
( )
2
2 3 7 2
2
8 3
7 2
1 10 7.85 10
4 4
5 10
1.7 10 1.08 10
7.85 10
A D m
l
R
A
m
R m
m
π π
ρ
− −
−
− −
−
= = × = ×
=
×
= × Ω = × Ω
×

8. A strain gauge with original resistance of and gauge factor F= 2 is installed on a
specimen with Young’s modulus Nm-2 and maximum stress
Determine the maximum change in resistance for the gauge.
( )( )
4
max
max 6
4 2
50
2.5 10
200 10
/
2 2.5 10 120 6 10
E
Maximum strain
E
R R
F
R F R
σ
ε
σ
ε
ε
ε
−
− −
=
= = = ×
×
∆
=
∆ = = × = × Ω
120
R
= Ω
6
200 10
× 2
max 50Nm
σ −
=

9. Specific
Adhesive
(glue) is used
to fix the
backing
material to
sample surface

10. Selection and Installation Factors for Bonded Metallic Strain Gages
• Grid material and configuration
• Backing material
• Bonding material and method
• Gage protection
• Associated electrical circuitry

11. Desirable Properties of Grid Material
• High gage factor, F
• High sensitivity
• Low temperature sensitivity
• High electrical stability
• High yield strength
• High endurance limit
• Good solderability or weldability
• Low hysteresis
• Low thermal emf when joined to other materials
• Good corrosion resistance

12. Properties of Common Grid Materials

13. Common Backing Materials
• Thin paper
• Phenolic-impregnated paper
• Epoxy-type plastic films
• Epoxy-impregnated fiberglass
• Most foil gages use an epoxy film backing

14. Bonding Procedure
• Select Strain Gauge
The two primary criteria for selecting the right type of
strain gauge are sensitivity and precision. So Select the
strain gauge model and gage length which meet the
requirements of the measuring object and purpose
• Remove Dust and Paint
Using a sand cloth polish the strain-gage bonding site
over a wider area than the strain-gage size. Wipe off
paint, rust and plating, if any, with a grinder or sand
blast before polishing
• Decide Bonding Position
Using a pencil or a marking-off pin, mark the
measuring site in the strain direction. When using a
marking off pin, take care not to deeply scratch the
strain-gage bonding surface

15. Bonding Procedure
• Remove grease from bonding surface and clean
Using an industrial tissue paper (SILBON paper)
dipped in acetone, clean the strain-gage bonding site.
Strongly wipe the surface in a single direction to
collect dust and then remove by wiping in the same
direction. Reciprocal wiping causes dust to move back
and forth and does not ensure cleaning
• Apply adhesive
Ascertain the back and front of the strain gage. Apply a
drop of adhesive to the back of the strain gage. Do not
spread the adhesive. If spreading occurs, curing is
adversely accelerated, thereby lowering the adhesive
strength

16. Bonding Procedure
• Bond strain gage to measuring site
After applying a drop of the adhesive, put the strain gage on
the measuring site while lining up the center marks with the
marking off lines
• Press strain gage
Cover the strain gage with the accessory polyethylene sheet
and press it over the sheet with a thumb. Once the strain gage
is placed on the bonding site, do not lift it to adjust the
position
• Complete bonding work
After pressing the strain gage with a thumb for one minute or
so, remove the polyethylene sheet and make sure the strain
gage is securely bonded. The above steps complete the
bonding work. However, good measurement results are
available after 60 minutes of complete curing of the adhesive

17. Some Adhesives and Their Preferred Curing Time

18. Applications of strain Gauges
Strain gauges are basically strain transducers which converts the mechanical signals into
electrical signals and hence measure the strain produced. This strain can be utilized further
to measure the following quantities as given:
• Force
• Torque
• Pressure
• Flow Rate
• Bending Stress

19. Measurement of Force
• Force can be measured using strain gauge load cells
• A load cell is a transducer that is used to convert a force into electrical signal
• A load cell is made by bonding strain gauges to a spring material. To efficiently detect
the strain, strain gauges are bonded to the position on the spring material where the
strain will be the largest
• Two gauges are along the direction of
applied load and other two are at right
angle to these.
• When there is no load, all gauges have
same resistance and bridge is balanced
• When load is applied, there is change in
resistance and hence some output voltage
is there which is the measure of applied load.
Tension-compression resistance strain-gage load cell

21. A strain gauge with original gauge factor F= 2 is installed on a rectangular beam with
Young’s modulus kNm-2. The beam is 3cm in width and 1cm thick, and a tensile
force of 30kN is applied axially. Determine the change in resistance. Given that the
resistance of the gauge during no-load is 120Ω.
6
200 10
×

22. Pressure Measurement
• Use elastic diaphragm as primary pressure transducer
• Apply strain gage directly to a diaphragm surface and calibrate the measured strain in
terms of pressure
• Pressure is measured through force that is exerted on the diaphragm where the force will
be detected by the strain gauge and resistance change will be produced
Location of strain gages on
flat diaphragm
The central gage is subjected
to tension while the outer
gage senses compression

24. Flow Measurement

27. Torque Measurement
• Four bonded-wire strain gauges are mounted on a 450 helix with axis of rotation and
place in pairs diametrically opposite as shown in figure
• If gauges are accurately placed and have matched characteristics, the system is
temperature compensated and insensitive to bending, thrust or pulls
• Any change in resistance is purely due to torsion of shaft, hence the torque can be
determined by measuring change in voltage which can be written as

28. Bending Stress with Cantilever beam

31. Wheatstone Bridge

35. Temperature Effects and Need for Temperature Compensation
Measurements are performed with strain gauges in mechanical stress analysis to examine
loading and fatigue. In addition to the desired measurement signal indicating mechanical
strain, each strain gauge also produces a temperature-dependent measurement signal. This
signal, called the apparent strain, is superimposed on the actual measured value.
Various effects contribute to the apparent strain:
• Thermal expansion of the measurement object (i.e. strain due entirely to temperature with
no mechanical loading as the cause)
• Temperature-dependent change in the strain gauge resistance
• Thermal contraction of the strain gauge measuring grid foil
• Temperature response of the connection wires

37. Methods For Temperature Compensation
 Active Dummy Method
• The active-dummy method uses the 2-gage system where an active gage, A, is bonded
to the measuring object and a dummy gage, D, is bonded to a dummy block which is
free from the stress of the measuring object but under the same temperature condition
as that affecting the measuring object. The dummy block should be made of the same
material as the measuring object.
• As shown in Fig, the two gages are connected to adjacent sides of the bridge. Since the
measuring object and the dummy block are under the same temperature condition,
thermally-induced elongation or contraction is the same on both of them. Thus, gages
A and B bear the same thermally-induced strain, which is compensated to let the
output, e, be zero because these gages are connected to adjacent sides.

39. Protecting the Strain Gage
• The strain gages must be protected from ambient conditions e.g. moisture, oil, dust and
dirt
• Protective materials used are Petroleum waxes, silicone resins, epoxy preparations,
rubberized brushing compounds

40. Possible sources of error in strain gauge signals
1- Cross-sensitivity
Because a strain gauge has width as well as length, a small proportion of the resistance
element lies at right angles to the major axis of the gauge, at the points where the conductor
reverses direction at the ends of the gauge. So as well as responding to strain in the direction
of its major axis, the gauge will also be somewhat responsive to any strain there may be at
right angles to major axis.
2- Bonding faults
For perfect bonding, the suitable adhesives and procedures for bonding gauges to the strain
surface should be complied. If the bonding is unsatisfactory, creep may occur. Creep is a
gradual relaxing of the strain on the strain gauge, and it has the effect of decreasing the gauge
factor, so that the output of the bridge becomes less than it should be. Creep may also occur
where gauges have been used to measure dynamic strain, and have been subjected to many
thousands of cycles of strain.

41. 3- Hysteresis
If a strain gauge installation is loaded to a high value of strain and then unloaded, it may be
found that the gauge element appears to have acquired a permanent set, so that resistance
values are slightly higher when unloading. The same effect continues when the direction of
loading is reversed. To manipulate this problem, repeating cycles of loading/unloading
should cause the hysteresis loop to narrow to negligible
4- Effects of moisture
The gauges or the bonding adhesive may absorb water. This can cause dimensional changes
which appear as false strain values. Another effect when moisture connections forms high
resistance connected in parallel with the gauge. To prevent this, gauges should be bonded in
dry condition or a suitable electrically insulating water repellent, such as a silicone rubber
compound.
5- Temperature change
One possible source of temperature difference is the heat produced by the current through a
strain gauge. When the bridge is first switched on, the gauges may warm up, so the bridge
should not be used for measurement until sufficient time for temperature to stabilize.

42. 1. Understand the basic operating principle of a strain gauge
2. Able to install a strain gauge with correct procedure
3. Able to apply strain gauge for different measurement
procedures.
4. Able to understand and apply Wheatstone bridge circuit for
strain gauge measurement